Relatively new methods of producing metallic iron by heat-reducing an iron oxide source such as an iron ore or the like include a method of producing metallic iron comprising reducing iron oxide by heating, on a moving hearth, a mixed power containing an iron oxide source such as iron ore and a carbonaceous reducing agent such as a carbonaceous material, or a carbonaceous material-containing raw material agglomerated by pelletizing the mixture.
In carrying out this method, a known method is performed, in which in order to increase a reduction potential on the hearth for heating-reduction to improve a reduction efficiency, an atmosphere control carbonaceous material is spread on the hearth before the raw material is charged (for example, Japanese Unexamined Patent Application Publication Nos. 11-106816, 11-106816, 11-172312, 11-335712, 2000-45008, etc.). The atmosphere control carbonaceous material is confirmed to effectively function to prevent direct contact between a hearth refractory and metallic iron and generated slag, which are produced by heating-reduction, and to suppress corrosion of the hearth refractory.
As a result of the advance of research on a method of producing metallic iron by using an atmosphere control carbonaceous material, the inventors found that the above-described conventional methods have the unsolved problems below.
The greatest problem pointed out in the conventional methods is that a powder and granular carbonaceous material used for controlling an atmosphere is fused and solidified into a rice-cracker-like shape in the step of heat-reducing the iron oxide-containing raw material to cause warping, depending on the type of the powder and granular carbonaceous material used, thereby significantly hindering a continuous operation. When such a phenomenon occurs on the hearth during an operation, the following various problems are caused.
(1) The metallic iron and generated slag produced by heating-reduction are generally solidified by cooling on the lowermost stream side of a production apparatus, and then discharged from the hearth by using a scraper device such as a screw or the like. However, the warped carbonaceous material resolidified into a rice-cracker-like shape is caught by the scraper device, thereby significantly inhibiting discharge of the metallic iron and generated slag from the hearth.
(2) When the carbonaceous material re-solidified into a rice-cracker-like shape is forcedly discharged from the hearth by using the scraper device, a large load is applied to the scraper device to cause a failure in the device. Also, the hearth refractory is damaged by the re-solidified carbonaceous material to significantly deteriorate durability.
(3) The metallic iron produced by reduction is partially contained in the re-solidified carbonaceous material, thereby deteriorating a recovery ratio of the metallic iron.
(4) When a resolidificable carbonaceous material is spread on the hearth before a raw material is charged, the carbonaceous material is re-solidified into a rice-cracker-like shape to cause warping. Therefore, when a raw material is supplied on the re-solidified carbonaceous material, the raw material flows toward a lower portion or drops into a crack of the carbonaceous material layer, thereby failing to charge the raw material in a uniform thickness.
Furthermore, most of the carbonaceous material discharged from the hearth still has high reduction activity. However, in the conventional technique, the carbonaceous material is discarded with substantially no further treatment, leaving room for improvement from the viewpoint of effective utilization of valuable resources.
The present invention have been achieved in consideration of the above-described situation, and an object of the present invention is to resolve the above various problems due to the rice-cracker-like carbonaceous material produced by re-solidification of a powder and granular carbonaceous material used for controlling an atmosphere. Another object of the present invention is to establish a technique for effectively recycling the used carbonaceous material still having reduction activity as a variable resource, to decrease the consumption of the atmosphere control carbonaceous material.